In recent years, as information technology has been advanced, there has been an increasing demand for flat panel displays. Examples of known flat panel displays include a non-light emitting type liquid crystal display (LCD), a light emitting type plasma display (PDP), an inorganic electroluminescence (inorganic EL) display, an organic EL display, etc. Among these flat panel displays, there has been particularly a significant advance in the organic EL display.
An organic EL element, which has received attention as a next-generation light emitting element, includes at least an emitting layer made of an organic material between a pair of electrodes, and further optionally includes a charge injecting layer for injecting electronic charge into the emitting layer and a charge transporting layer for transporting electronic charge from the electrode to the organic layer. The organic EL element can have a thinner thickness and a lighter weight, and has properties, such as low voltage drive, high luminance, light emission, etc., and therefore, is currently extensively studied and developed.
The organic EL element is expected to be applied not only to displays, but also to light sources for electrophotographic copiers, printers, etc., and illumination. Organic EL illumination devices including the organic EL element for illumination have many advantages, such as surface emission, high color rendering, ease of dimming, free of mercury, which is contained in fluorescent lamps, ultraviolet-free light, etc.
The organic EL element is a light emitting element which is based on the principle that a fluorescent material emits light which is generated by recombination energy between holes injected from the anode and electrons injected from the cathode in the presence of applied electric field. Organic EL elements made of organic materials have been extensively studied since the report of a low voltage drive organic EL element by C. W. Tang et al. (see NON-PATENT DOCUMENT 1).
In the organic EL element, however, 80% or more of applied energy is dissipated as heat. Also, in the organic EL element, the ease at which a current flows increases exponentially with an increase in temperature. This local temperature increase leads to concentration of a current in the higher temperature portion, disadvantageously resulting in luminance unevenness due to a local increase in emission intensity or luminance unevenness due to degradation or damage of the organic EL element caused by a further temperature increase.
Therefore, for the organic EL element, a technique of efficiently dissipating heat generated during light emission has been proposed.
More specifically, an organic EL illumination device has been described in which heat dissipating means for dissipating heat of a substrate provided on a side opposite to the light emission side is provided on an outer surface of the substrate. The heat dissipating means includes any of a heat dissipation fin, a heat pipe, and a Peltier element. With this configuration, a simple or low-cost structure can be used to efficiently perform heat dissipation of the organic EL element (see, for example, PATENT DOCUMENT 1).